“Plato, despair!
We prove by norms
How numbers bear
Empiric forms,

How random wrong
Will average right
If time be long
And error slight,

But in our hearts
Hyperbole
Curves and departs
To infinity.

Error is boundless.
Nor hope nor doubt,
Though both be groundless,
Will average out.”

Source: Shakespeare's Sonnets

Statement on the doorstep of 10 Downing Street, after her election as Prime Minister, as quoted at On this day (BBC) http://news.bbc.co.uk/onthisday/hi/dates/stories/may/4/newsid_2503000/2503195.stm. (This is a paraphrasing of a prayer commonly misattributed to St. Francis of Assisi.)
First term as Prime Minister
Source: [Who wrote Prayer of St. Francis? Doubtful it was friar, San Diego Union-Tribune, 27 January 2009, 28 July 2019, https://www.sandiegouniontribune.com/sdut-1n27prayer00320-who-wrote-prayer-st-francis-doubtf-2009jan27-htmlstory.html, Lord, make me an instrument of thy peace. Where there is hatred, let me sow love; Where there is injury, pardon; Where there is doubt, faith; Where there is despair, hope; Where there is darkness, light; Where there is sadness, joy.]
Source: [The real prayer of Francis of Assisi, Howse, Christopher, The Daily Telegraph, 12 April 2013, 28 July 2019, https://www.telegraph.co.uk/news/religion/9991301/The-real-prayer-of-Francis-of-Assisi.html, That was written in 1912, in French, and published in a pious magazine edited by Fr Esther Bouquerel. It was attributed to St Francis in 1927 through its having been printed on the back of a picture of the saint.]
Source: [Who wrote Prayer of St. Francis? Doubtful it was friar, San Diego Union-Tribune, 27 January 2009, 28 July 2019, https://www.sandiegouniontribune.com/sdut-1n27prayer00320-who-wrote-prayer-st-francis-doubtf-2009jan27-htmlstory.html, An article published last week in L'Osservatore Romano, the Vatican newspaper, said the prayer in its current form dates only from 1912, when it appeared in a French Catholic periodical. ... Although news to many, the truth about the prayer had apparently been hiding in plain sight. “No one among the Franciscans ever thought it really was by St. Francis,” said Giovanni Maria Vian, the editor of L'Osservatore Romano.]

Letter https://founders.archives.gov/documents/Jefferson/98-01-02-1712 to William Roscoe (27 December 1820)
1820s

To Cyriack Skinner, upon His Blindness (c. 1655)

Part I, line 417.
The Grave (1743)

Source: Derek Parfit, ‘Reasons and Motivation’, Proceedings of the Aristotelian Society, supp. vol. 71 (1997), p. 121

Source: Nietzsche: Philosopher, Psychologist, Antichrist (1951), p. 151
Context: There is thus a certain plausibility to Nietzsche's doctrine, though it is dynamite. He maintains in effect that the gulf separating Plato from the average man is greater than the cleft between the average man and a chimpanzee.

Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

The Origins of Knowledge and Imagination (1978)